The invention is based on patent application Nos. 2000-199023 Pat., 2000-236810 Pat. 2001-72054 Pat., and 2001-72911 Pat. filed in Japan, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display element, and in other words, a liquid crystal light modulation element and method of producing the same.
2. Description of the Background Art
The liquid crystal display element and, in other words, the liquid crystal light modulation element primarily includes a pair of substrates, between which a liquid crystal layer including liquid crystal material is held. For example, predetermined drive voltage is applied to the liquid crystal layer to control orientation of liquid crystal molecules in the liquid crystal layer so that external light incident on the liquid crystal light modulation element is modulated to perform intended display of images or the like.
The liquid crystal light modulation element using the cholesteric liquid crystal has been known as the above kind of liquid crystal light modulation element, and various studies have been made.
Examples of the cholesteric liquid crystal are, e.g., liquid crystal, which exhibits the cholesteric phase by itself, and chiral nematic liquid crystal obtained by adding a chiral agent to nematic liquid crystal.
The cholesteric liquid crystal has such a feature that the liquid crystal molecules form helical structures, and can exhibit three states, i.e., a planar state, focal conic state and a homeotropic state when it is held between a pair of substrates, and is subjected to an external stimulus such as an electric field, a magnetic field or a heat.
In the liquid crystal light modulation element (e.g., liquid crystal display element) using the cholesteric liquid crystal, these three states exhibit different light transparencies and reflectances. Therefore, the three states and the manner of applying the external stimulus can be appropriately selected to perform the display. For example, the display may be performed in the cholesteric-nematic phase transfer mode using the homeotropic state and the focal conic state, and may be performed in a bistable mode using the planar state and the focal conic state.
Among them, the display in the bistable mode has such a feature that the planar state and the focal conic state are stable even in the state where no external stimulus is applied, and thus has the bistability (memory property), which maintains the display state even when no external stimulus (e.g., voltage) is applied. For the above reason, the liquid crystal light modulation element using the cholesteric liquid crystal has been increasingly studied in recent years as the memorizable element (display element achieving the stable display state).
In particular, the liquid crystal light modulation element using the cholesteric liquid crystal, which exhibits the selective reflection property in the visible wavelength range when it is in the planar state, has the memorizable property, and can achieve a bright reflection state. In other words, it can perform bright display without using a polarizing plate or a color filter. Therefore, it is expected that the liquid crystal light modulation element described above can be used as a display element, which is very effective at reducing the power consumption, and can be used as a display element of, e.g., a mobile telephone requiring low power consumption.
The liquid crystal having the bistability can be stable in both the planar state (i.e., the state of the planar orientation), where the helical axis of the cholesteric liquid crystal is substantially perpendicular to the substrate surface, and the liquid crystal exhibits the selective reflection state, and the focal conic state (the state of the focal conic orientation), where the helical axis of the liquid crystal is substantially parallel to the substrate surface, and the liquid crystal is transparent to the visible light.
However, in the liquid crystal display element utilizing the selective reflection characteristics of the cholesteric liquid crystal, the reflection wavelength shifts toward the shorter side in accordance with the incident angle of the light and observation angle because it employs the reflection manner using the light interference.
This phenomenon becomes more remarkable as the helical axis of the cholesteric liquid crystal in the planar orientation is closer to the vertical direction to the substrate surface. In particular, a TN liquid crystal element and an STN liquid crystal element may use a pair of substrates having deposited and rubbed polyimide thin films thereon for holding a liquid crystal layer therebetween, in which case the helical axis of the cholesteric liquid crystal is perfectly or substantially perfectly perpendicular to the substrate surface, resulting in an extremely narrow view angle. If the above liquid crystal element is used as the display element, therefore, the viewability becomes extremely low.
The rubbing of the thin polyimide film increases the restricting force on a polyimide interface so that it becomes difficult to maintain the focal conic state. Consequently, the bistability, which is the distinctive feature of the cholesteric liquid crystal, may be lost.
For avoiding the above, it has been attempted to incline slightly the helical axis of the cholesteric liquid crystal with respect to the normal of the substrate. One of such attempts is called PSCT (Polymer Stabilized Cholesteric Texture), in which polymers are dispersed in the cholesteric liquid crystal so that the helical axes may be positioned in random directions owing to mutual operations between the polymers and the liquid crystal (U.S. Pat. No. 5,384,067). According to this method, however, mixing of the polymer in the liquid crystal material may lower the reliability of the element, and/or may require the increased drive voltage.
In another method, a polyimide film not subjected to the rubbing is deposited on substrate surface opposed to the liquid crystal so that the helical axis may be inclined. In this method, however, domains including different directions of the inclined helical axes (directions of the helical axes projected onto the substrate) are formed randomly so that scattering of the incident light is liable to occur due to the difference in refractive index between the domains, resulting in lowering of the purity of the display color in the selective reflection. In a multilayer liquid crystal display element employing a multilayer structure for multicolor display, the reflection light from the lower layer is liable to be affected by light scattering by an upper layer, which lowers both the contrast and color purity.
For improving the characteristics of the cholesteric liquid crystal element, in which the liquid crystal is held between the substrates provided with the polyimide films not subjected to the orientation processing, Japanese Laid-open Patent Publication No. 10-31205 (31205/1998) has disclosed the following manner. Different surface treatments are effected on the polyimide films formed on the substrates on the observation side and the non-observation (opposite) side, respectively. More specifically, the rubbing processing is effected on only the polyimide film on the non-observation side, and the liquid crystal domains on the observation side may be the non-orientation random domains (polydomain state). Thereby, the helical axes of the liquid crystal on the non-observation side may be substantially perfectly perpendicular to the substrate surface, and the liquid crystal domains on the non-observation side may be uniform (mono-domain state).
According to this manner, however, the rubbing is effected on the whole polyimide film area of the substrate on the non-observation side. Therefore, the liquid crystal domains form the monodomain state on the whole substrate so that the stability in the focal conic state is liable to lower, and the bistability, which is the feature of the cholesteric liquid crystal element, is may be impaired. In the planar orientation state, the inclination of helical axes of the liquid crystal on the random domain side is gradually lost, which impairs the long-term bistability. In any one of the above case, it is difficult to maintain the display state (good display state with high contrast and color purity) for a long time without voltage application, and it is difficult to achieve the intended characteristics for high contrast and high color purity together with the bistability.
In the focal conic state of the cholesteric liquid crystal, the helical axes of liquid crystal molecules are parallel to the substrate plane. Usually, the liquid crystal has a plurality of liquid crystal molecule regions (liquid crystal domains). In the focal conic state, the helical axes of the liquid crystal are parallel or substantially parallel to each other in each liquid crystal domain, but the directions Fxe2x80x2 of the helical axes in the neighboring liquid crystal domains are not parallel to each other as shown in FIG. 29. Accordingly, due to the difference in refractive index between the liquid crystal domains, the light incident on the liquid crystal element is slightly scattered at an interface between the liquid crystal domains. In particular, if the helical pitch is small (more specifically, if the helical pitch of the liquid crystal in the planar state is small to cause the selective reflection in the visible range), the liquid crystal domains become small in principle, and the light scattering occurs to a large extent in the element so that employment thereof in the display element cause low contrast.
It is also known to use an element (multilayer liquid crystal element) formed of a plurality of liquid crystal layers stacked together and, e.g., having different selective reflection wavelengths, respectively, for providing a multilayer liquid crystal light modulation element, which allows color display in two or more colors (e.g., full color display). In the case of this multilayer structure, multiple-scattering or the like between the liquid crystal layers particularly increases the influence due to the scattering between the domains so that the contrast is liable to be low.
In the display region of the liquid crystal display element (liquid crystal light modulation element), electrodes are not located on the opposite sides of the liquid crystal in the region other than the pixels, and thus, the non-pixel region (the inter-pixel region). Therefore, the molecules of the liquid crystal in such region cannot be controlled. This results in the following disadvantage.
If the liquid crystal between the substrates is in the planar state (e.g., in the case where a multilayer liquid crystal display element is to be formed by stacking and adhering the plurality of liquid crystal display elements under a pressure, and particularly the liquid crystal between the substrates in each liquid crystal display element is in the planar state due to the pressure), a predetermined voltage may be applied to the liquid crystal of the pixel(s) in one or more liquid crystal display elements for changing the liquid crystal in the pixel(s) into the focal conic state, whereby the molecular orientation of the liquid crystal of the pixel(s) is controlled to attain the focal conic state, as shown in FIG. 5. However, the liquid crystal between the neighboring pixels is affected by the applied voltage, and thereby partially attains the focal conic state so that the focal conic state and the planar state are mixed in the liquid crystal between the pixels. In this mixed state, the domains of the different state may be adjacent to each other. In general, as compared with the case of only the planar state alone, the domains are small in the case where the two states are mixed, and therefore incident light is liable to scatter. Further, selective reflection of the incident light may partially occur.
In the liquid crystal display element, a predetermined voltage may be applied to the liquid crystal of the pixel for changing it from the focal conic state to the planar state. In this case, as shown in FIG. 6, the molecular orientation of liquid crystal of the pixel is controlled to attain the planar state. However, the liquid crystal between the neighboring pixels is affected by the applied voltage to attaint partially the planar state. Thus, the planar state and the focal conic state are mixed in the liquid crystal between the pixels.
For the above reasons, the planar state and the focal conic state are mixed in the liquid crystal between the pixels in the liquid crystal display element. In FIGS. 5 and 6, S indicates the substrate, T indicates the electrode, Lc indicates the liquid crystal molecules, P indicates the planar orientation state of the liquid crystal molecules, and F indicates the focal conic orientation state of the liquid crystal molecules.
As described above, a part of the incident light is selectively reflected and scattered by the liquid crystal between the pixels due to mixing of the focal conic state and the planar state of the liquid crystal between the pixels. This deteriorates the display characteristics of the liquid crystal display element.
According to the study by the inventors, if the rubbing processing is not effected on the substrate surface or the like for controlling the orientation directions of the liquid crystal molecules in the liquid crystal display element of the reflection type, the liquid crystal molecules between the substrates tend to be positioned in the random directions so that the view angle range allowing good observation of the display can be increased. This is already known.
However, if the rubbing processing is not effected for increasing the view angle, the liquid crystal molecules between the pixels are positioned in random directions. Therefore, the liquid crystal between the pixels forms small domains, and light scattering is liable to occur on the boundary between the domains.
As described above, in the liquid crystal display element or in the multilayer liquid crystal display element formed of the plurality of liquid crystal layers stacked together, the incident light may be scattered or selectively reflected (R1 in FIG. 7) if the light is applied to the liquid crystal between the pixels in each liquid crystal display element without effecting no control on the molecular orientation, as shown in FIG. 7.
In the multilayer liquid crystal display element Axe2x80x2, as shown in FIG. 7, the liquid crystal in the non-pixel region on the upper side (image observation side), i.e., the liquid crystal in the regions between the pixels scatters the light, which is selectively reflected by the liquid crystal display element lower than the liquid crystal display element nearest to the observation side, and passes toward the observation side (R2 in FIG. 7).
In this state, when performing the color display using the stacked liquid crystal display elements for display in red, green and blue, respectively, white display can be performed with high brightness owing to the selective reflection and scattering by the liquid crystal in the non-pixel domains. However, when performing, e.g., the black display by a light absorbing layer Bk in the focal conic state of the liquid crystal in the pixels, the black display is blurred due to the selective reflection and scattering of the incident light by the liquid crystal between the pixels, resulting in low contrast of the image display. Further, since the selective reflection and scattering of the incident light are caused by the liquid crystal between the pixels, and the liquid crystal between the pixels scatters the light, which is selectively reflected by the lower layer toward the observation side, these lower the color purity in display.
In any one of the above cases, the optimum solution has not yet achieved in connection with the orientation control of the liquid crystal in the above types of liquid crystal display element.
A primary object of the invention is to provide a liquid crystal display element capable of image display with high quality.
Another object of the invention is to provide a liquid crystal display element capable of image display with high contrast.
Still another object of the invention is to provide a liquid crystal display element capable of image display with good color purity.
Yet another object of the invention is to provide a method of producing such an improved liquid crystal display element.
The invention provides the following liquid crystal display elements (liquid crystal light (optical) modulation elements) and methods of producing the same.
(1) Liquid Crystal Display Element (Liquid Crystal Light (Optical) Modulation Element)
(1-1) First Element
A liquid crystal display element including a liquid crystal layer including liquid crystal contained between a pair of substrates and exhibiting a cholesteric phase, wherein
an orientation film is arranged on at least one of the paired substrates, and is in contact with the liquid crystal layer, and liquid crystal molecular orientation processing for portions of each orientation film corresponding to pixel regions is effected in a manner different from that effected on at least a portion of a portion corresponding to non-pixel region (inter-pixel region) of the orientation film on at least one of the substrates.
The invention also provides a multilayer liquid crystal display element formed of the plurality of first liquid crystal display elements stacked together.
(1-2) Second Element
A liquid crystal display element including a liquid crystal layer arranged between a pair of substrates and including liquid crystal exhibiting a cholesteric phase, and a plurality of pixels, wherein an orientation film is formed on at least one of the substrates, and liquid crystal molecular orientation processing is effected on at least a portion of a portion corresponding to non-pixel region (inter-pixel region) of the orientation film.
The invention also provides a multilayer liquid crystal display element formed of the plurality of second liquid crystal display elements stacked together.
(1-3) Third Element
A liquid crystal display element formed of a plurality of liquid crystal layers stacked together and each held between a pair of substrates, wherein at least one of the plurality of liquid crystal layers is provided with an orientation film arranged on at least one of the paired substrates holding the liquid crystal layer therebetween and being in contact with the liquid crystal layer, and liquid crystal molecular orientation processing for portions of each orientation film corresponding to pixel regions is effected in a manner different from that effected on at least a portion of a portion corresponding to non-pixel region (inter-pixel region) of the orientation film on at least one of the substrates.
The invention further provides fourth and fifth elements as well as first and second element producing methods described later. These are based on the following findings of the inventors.
In the liquid crystal light(optical) modulation element including a pair of substrates and a liquid crystal layer held between the substrates and including a liquid crystal material, which exhibits a cholesteric phase in a room temperature and has a peak of a selective reflection wavelength in a visible wavelength range, a mixed state of a polydomain state and a monodomain state may be attained in the liquid crystal domains of the pixel regions near at least one of the substrates holding the liquid crystal layer in the selective reflection state. Alternatively, the polydomain state may be achieved in each of the liquid crystal domains of the pixel regions near the substrates of the liquid crystal layer in the selective reflection state. Thereby, the liquid crystal in the liquid crystal domains in the pixel region near one of the opposite substrates may have the cholesteric helical axes different in angle with respect to a normal of the substrate from that of the other substrate. Thereby, the reflected light can be collected on the front surface on the element observation side, and the good image display with high brightness, contrast and color purity can be performed. Further, when no external stimulus (e.g., no voltage) is applied, the display state (image display with high brightness, contrast and color purity) can be maintained for a long term.
The above xe2x80x9cpolydomain statexe2x80x9d is a bunch of domains, where the helical axes of the liquid crystal in the selective reflection state are slightly inclined with respect to the substrate normal, and the directions of the helical axes projected on the substrate are randomly different among the domains. The xe2x80x9cmonodomain statexe2x80x9d is a bunch of domains where the helical axes of the liquid crystal are perpendicular or substantially perpendicular to the substrate surface, and thus extend in a uniform direction.
(1-4) Fourth Element
A liquid crystal light(optical) modulation element including a liquid crystal layer held between a pair of substrates and including a liquid crystal material exhibiting a cholesteric phase in a room temperature and having a peak of a selective reflection wavelength in a visible wavelength range, wherein
the liquid crystal layer in a selective reflection state has pixel regions neighboring to the opposite substrates, respectively, and liquid crystal domains in the pixel regions neighboring to at least one of the substrates are in a mixed state of a polydomain state and a monodomain state.
(1-5) Fifth Element
A liquid crystal light(optical) modulation element including a liquid crystal layer held between a pair of substrates and including a liquid crystal material exhibiting a cholesteric phase in a room temperature and having a peak of a selective reflection wavelength in a visible wavelength range, wherein
the liquid crystal layer in a selective reflection state has pixel regions neighboring to the opposite substrates, respectively, each of liquid crystal domains in the pixel regions take a polydomain state, and angles of the cholesteric helical axes of the liquid crystal with respect to the substrate normal are different between the liquid crystal domains in the pixel regions near one of the opposite substrates and the liquid crystal domains in the pixel regions near the other substrate.
The invention also provides a liquid crystal light (optical) modulation element, in which a plurality of liquid crystals each held between a pair of substrates are stacked, and at least one of the plurality of liquid crystal layers forms together with the corresponding pair of substrates holding the liquid crystal layer said fourth or fifth liquid crystal optical modulation element.
The inventors have also found such a phenomenon that scattering between the domains is remarkably reduced by aligning the directions of the helical axes of the cholesteric liquid crystal molecules in the focal conic state, and provides a sixth element and a third element producing method described later based on the above finding.
(1-6) Sixth Element
A liquid crystal light(optical) modulation element for performing light (optical) modulation by utilizing a focal conic state of liquid crystal molecules included in a liquid crystal layer held between a pair of substrates, wherein helical axes of the liquid crystal molecules in the focal conic state extend in regular directions within a plane substantially parallel to a substrate surface.
As an element of the same kind as the above, the invention provides a liquid crystal light(optical) modulation element for performing light(optical) modulation by utilizing a focal conic state of liquid crystal molecules included in a liquid crystal layer held between a pair of substrates, wherein orientation regulating means for the liquid crystal molecules is employed for orientating the helical axes of the liquid crystal molecules in the focal conic state in regular directions within a plane substantially parallel to a substrate surface.
The invention also provides a multilayer liquid crystal display element formed of the plurality of said liquid crystal optical modulation elements stacked together.
(2) Method of Producing Liquid Crystal Display Element (Liquid Crystal Light(Optical) Modulation Element)
(2-1) First Element Producing Method
A method of producing a liquid crystal light(optical) modulation element including a liquid crystal layer held between a pair of substrates and including a liquid crystal material exhibiting a cholesteric phase at a room temperature and having a peak of a selective reflection wavelength in a visible wavelength range, including:
a substrate processing step of processing at least one of the paired substrates such that the liquid crystal layer in the selective reflection state has pixel regions neighboring to the opposite substrates, respectively, and liquid crystal domains in the pixel regions neighboring to at least one of the substrates are in a mixed state of a polydomain state and a monodomain state; and
a step of arranging the liquid crystal layer between the paired substrates including the substrate(s) subjected to the substrate processing step.
(2-2) Second Element Producing Method
A method of producing a liquid crystal light(optical) modulation element including a liquid crystal layer held between a pair of substrates and including a liquid crystal material exhibiting a cholesteric phase at a room temperature and having a peak of a selective reflection wavelength in a visible wavelength range, including:
a substrate processing step of processing the paired substrates such that the liquid crystal layer in the selective reflection state has pixel regions neighboring to the opposite substrates, respectively, liquid crystal domains in the pixel regions take a polydomain state, and the angles of the cholesteric helical axes of the liquid crystal with respect to the substrate normal are different between the liquid crystal domains in the pixel regions near one of the opposite substrates and the liquid crystal domains in the pixel regions near the other substrate; and
a step of arranging the liquid crystal layer between the paired substrates subjected to the substrate processing step.
(2-3) Third Element Producing Method
A method of producing a liquid crystal light(optical) modulation element for performing light (optical) modulation by utilizing a focal conic state of liquid crystal molecules included in a liquid crystal layer held between a pair of substrates, including the steps of providing orientation regulating means (e.g., a projected structure, a groove in an electrode formed on the substrate, an insulating film having a groove and formed on the substrate, a region on the substrate having partially different orientation regulating force) for the liquid crystal molecules for orientating helical axes of the liquid crystal molecules in the focal conic state on at least one of the substrates; and a step of arranging the liquid crystal layer between the paired substrates.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.